A lens is a device usually formed from a piece of shaped glass or plastic that causes light to either converge and concentrate, or to diverge. One important use of lenses is as a prosthetic for the correction of visual impairments such as myopia, hyperopia and presbyopia. Other uses are in imaging systems such as a monocular, binoculars, telescopes, spotting scopes, telescopic gun sights, theodolites, microscopes, and cameras (photographic lens).
Lenses do not form perfect images; there is always some degree of distortion or aberration introduced by the lens that causes the image to be an imperfect replica of the object. Thus, aberrations result when the optical system misdirects some of the object's rays. There are several types of aberration that can affect image quality. Some aberrations occur when electromagnetic radiation of one wavelength is being imaged (monochromatic aberrations), and others occur when electromagnetic radiation of two or more wavelengths is imaged (chromatic aberrations).
Because distortion introduced by aberrations into an optical system significantly degrades the quality of the images on the image plane of such system, there are advantages to the reduction of those aberrations. Various techniques are often used to minimize the aberrations. One such technique involves the use of a wavefront aberrator.
Wavefront aberrators are particularly useful in eye glasses or contact lenses for use in correcting human eye sight. U.S. Pat. No. 6,989,938 (the '938 patent) describes a wavefront aberrator and methods for manufacturing it. The '938 patent describes how a unique refractive index profile can be created across a monomer layer by controlling the extent of curing of the monomer in different regions of the layer, thus creating a wavefront aberrator.
The '938 patent describes a method that allows one to achieve a unique refractive index profile through the creation of regions with varying degrees of cure. While this technology is very useful, it gives rise to problems with lens stability. Because of the varying degrees of cure, a concentration gradient of monomer between different regions exists which may cause the monomer to diffuse over time. As the monomer diffuses from one region to another, the refractive index profile changes. As a result, the refractive index profile at a later time can be substantially different than the desired original refractive index profile that was initially created.
Also, curing of the lesser cured regions after the desired refractive index profile is created can lead to changes in the profile due to reduction of the contrast in the refractive index between the lesser cured and more highly cured regions. Unwanted curing of the lesser cured regions over time can take place by, for example flood photopolymerization (or flood curing) which activates polymerization initiators. Flood curing is process whereby substantially uniform radiation (light) is applied to an uncured or partially cured material over time. The time period can be long or short. This causes cross-linking of previously uncured monomers, and refractive index increases in the previously lesser cured regions, bringing their refractive indices closer to those of the originally more cured regions. A reduction in refractive index contrast results in changes to the optical characteristics. Exposure to sunlight is an example of a scenario under which refractive contrast is diminished by flood photopolymerization.
Some of these issues could be addressed by optimizing storage conditions to prevent changes in the optical characteristics. A wavefront aberrator could be stored at freezing conditions to prevent (or at least significantly slow) diffusion of monomer between lesser cured and more highly cured regions. Low temperature would also retard the rate of thermal polymerization. Furthermore, a wavefront aberrator could be stored in dark conditions to prevent activation of the polymerization initiator. Unfortunately a need for specific storage conditions could limit the number of practical applications for the wavefront aberrator. For example, one specific application of a wavefront aberrator is in a form similar to glasses or contact lenses to correct the aberrations of the human eye. Because humans wear their glasses in a variety of conditions (hot/cold, humid/dry, sunny/dark), temperature and light exposure can not be easily controlled. Thus, additional methods for creating stable refractive index profiles are needed.
The subject invention successfully addresses the above-described disadvantages associated with the prior art and provides certain attributes and advantages that have not been realized by the prior art.